Backing pad for sanding discs



I i i I 48 42 14 s5 s4 I 24 37 39 y 2, 1970 A. A. HUTCHINS 3,510,992

BACKING PAD FOR SANDING DISCS Filed Aug. 18. 1967 2 Sheets-Sheet l v 4 6 mm;

INVENTOR. flLMA? A. Hu TC H/us May 12, 1970 A. A. HUTCHINS 3,510,992

I BACKING PAD FOR SANDING DISCS Filed Aug. 18, 1967 I 2 Sheets-Sheet 2 3m 24a Haj 14 440. 58 34a 56 64 'QQIGOQ.

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1 I Y a laq ez 51a 44a 46a INVENTOR. flL/WA A. HuTCH/NS United States Patent 3,510,992 BA'CKING PAD FOR SANDING DISCS Alma A. Hutchins, 49 N. Lotus Ave., Pasadena, Calif. 91107 Filed Aug. 18, 1967, Ser. No. 661,592 Int. Cl. B24d 17/00 US. Cl. 51-378 17 Claims ABSTRACT OF THE DISCLOSURE A power sander pad including a rotatable backing plate carrying a circular cushion formed of a closed pore resiliently deformable elastomeric material to which a sanding disc is removably secured by adhesive, with the cushion having a large number of projections formed of the elastomeric material and bearing against the disc, and also having a large number of recesses formed in the elastomeric material between the projections.

CROSS REFERENCE TO RELATED APPLICATION Certain features of the sander which is typically disclosed in the present application are covered in my copending application Ser. No. 661,593 filed Aug. 18, 1967, now Pat. No. 3,496,680 on Powered Revolving Abrasive Too BACKGROUND OF THE INVENTION This invention relates to improved power sander apparatus, and particularly to a unique pad unit which carries a sandpaper disc in a power sander.

The sander pads of the present invention are preferably circular and connectable centrally to a driving unit for rotation relative to a portion of that unit, and carry a circular sanding disc which is detachably secured to a surface of the pad by a suitable adhesive or cement. The adhesive is of a type enabling the sandpaper disc to be easily stripped from the pad after the disc has become worn, so that a new disc may be substituted for further sanding operations.

In prior sanders utilizing discs of this adhesively retained type, the sandpaper carrying pad has usually been formed of a cushion of open-pored resilient material permanently carrying a sheet of canvas on its forward face. The sandpaper disc is then secured to the canvas by the mentioned adhesive. This arrangement has had several disadvantages which have reduced the effectiveness with which a device employing such a pad has been able to perform a sanding operation, and which have also greatly limited the useful life of such a pad. For one thing, a sandpaper disc backed up by a pad of this type usually tends to accumulate rapidly, on the surface of the sandpaper, many particles of the material being sanded, with the result that the spaces between the abrasive particles of the sandpaper are quickly filled in in a manner destroying the effective abrasiveness of the disc. Further, the rate at which such a disc can sand a workpiece has been slower than would be desired. Additionally, the canvas of the pad is readily attacked and damaged by the lacquer thinner or other solvent which must be employed to remove or soften the sandpaper retaining adhesive. This solvent also tends to soften or otherwise adversely affect the adhesive which secures the canvas to the main cushioning material, as well as the open-pored resilient rubber or other material employed as the backing cushion for the canvas. As a result of all of these factors, the entire pad soon is damaged to an extent rendering it completely unusable, and requiring its replacement.

3,510,992 Patented May 12, 1970 A sander pad constructed in accordance with the present invention is adapted to adhesively carry a circular disc of sandpaper or the like with the disc being easily removable and replaceable many times without damage to the backing pad or any other portion of the device. In spite of this case of removal and replacement, the pad is completely effective in properly backing up the disc during a sanding operation, and in fact attains a highly unique multiple point backing result which in actual practice considerably increases the rate of cut attained by a sanding disc. That is, a disc carried by a pad of the present invention will sand into or through the material of a workpiece much more rapidly than a conventional canvas backed disc, even though the rate of rotation of the driving unit and all other factors remain unchanged. Further, the present backing pad renders the carried disc automatically self-cleaning in operation, so that the abrasive surface does not become filled with particles from the work part being sanded, and will retain its abrasive characteristics over a long period of time.

Structurally, a sander pad formed in' accordance with the invention includes a backing plate having a fastener by which it may be detachably connected to a driving unit for rotation relative to a portion of that unit, and a cushion carried by the plate and formed of a resiliently deformable elastomeric material having an irregularized surface to which the abrasive disc is directly adhesively secured. More specifically, this irregularized surface of the elastomeric material is shaped to form a large number of projections of that material which are adhered to the sanding disc by an appropriate adhesive, with recesses formed in the elastomeric material between the projections and relieved away from the disc. The material of this cushion part thus exerts force against the sandpaper primarily at the many localized areas of the projections, and not as effectively or at all at the intermediate recessed locations, so that the etfective pressure or force per unit area exerted at the projection areas against the sandpaper is considerably greater than in the case of a smoother non-irregularized backing surface. This increased effective force improves the cutting efliciency and rate of cut of the sandpaper through a workpiece. Further, the freedom for limited deflection of the sandpaper at the recessed areas, into and out of the recesses, relative to the less readily deflectable portions of the disc which are backed up by the projections, results in an automatic cleaning of the abrasive sheet in a manner which will be discussed in greater detail at a later point.

The elastomeric material is desirably a closed pore sponge material, for best results neoprene sponge, and by virtue of its closed pore condition is not readily attacked by a lacquer thinner or other solvent which may be part of the adhesive by which the sandpaper is secured to the pad, or which may be utilized to remove, soften or reactivate the adhesive.

BRIEF DESCRIPTION OF THE DRAWING The above and other features and objects of the invention will be better understood from the following detailed description of the typical embodiments illustrated in the accompanying drawing, in which:

FIG. 1 is a side view of a sander carrying a sandpaper backup pad embodying the invention;

FIG. 2 is taken on line 22 of FIG. 1;

FIG. 3 is an axial section on line 3-3 of FIG. 1;

FIG. 4 is a fragmentary perspective view of the irregularized front surface of the elastomeric cushion;

FIGS. and 6 are enlarged sections on line 55 of FIG. 4, in two different conditions of the device;

FIG. 7 is an exploded perspective of a variational P FIG. 8 is taken on line 88 or FIG. 7;

FIGS. 9 and 11 are sections taken on lines 99 and 1111 of FIG. 8; and

FIG. 10 is taken on line 10-10 of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, I have shown at 10 a power driven sanding tool to be utilized for driving a circular rotatable backing pad 11 constructed in accordance with the invention and carrying a conventional circular sandpaper disc 12. Tool 10 may have a handle portion 13 carrying a main body 14 within which there is contained a rotary motor 15 typically driven by air about an axis 16, under the control of a manually operated air supply valve 17 actuated by a trigger 18. The tool acts to drive a head 19 which detachably carries backing pad 11 by reception of a threaded screw or stud 20 projecting from the backing pad into a threaded passage 21 formed in bottom wall 22 of head 19.

It is contemplated that tool 10 may be the conventional type of disc sander in which pad 11 and the carried sandpaper disc 12 are driven rotatably about main axis 16 of the motor; or alternatively (and preferably) tool 10 may be of the somewhat different type disclosed and claimed in my above identified co-pending application Ser. No. 661,593. This latter arrangement is illustrated in the drawing, in which it will be noted that the driven shaft 23 of motor 15 has a lower eccentric portion 24 centered about an axis 25 which is offset laterally from but parallel to main axis 16 of the motor. This eccentric portion 24 of the shaft is connected to the inner race of a ball bearing assembly 26, whose outer race is rigidly connected to and rotatably mounts the head 19. Thus, the entire head 19 revolves bodily about axis 16 as motor 15 turns, and in accordance with the revolving motion of axis 25 of eccentric portion 24 of the shaft, while at the same time body 19 and the carried disc 12 and backup pad 11 all freely rotate about portion 2 4 of the shaft, so that in effect the pad 11 and disc 12 revolve bodily about axis 16 without simultaneously rotating about that axis or about axis 24. Thus, an improved type of sanding action is attained by virtue of the fact that the disc 12 does not rotate rapidly about the axis of pad 11.

With regard now to FIG. 3, it is noted that pad 11 may include two rigid circular discs 27 and 28, both centered about the main axis 24 of the pad (which axis coincides with the previously mentioned axis 24 of FIG. 1 when the pad and sanding disc are in use). Backing plates 27 and 28 may be rigidly and permanently secured together in any suitable manner, as by cement bonding these two plates together. Preferably, these plates are formed of an appropriate very strong and rigid resinous plastic material, such as a fiberglass reinforced phenolic. The outer circular edge 29 of backing plate 28 may be of a diameter somewhat greater than but concentric with the outer circular edge 30 of plate 27. Thus, these two plates 27 and 28 form together a rigid backing plate structure by which screws 20 is carried.

The screw 20 has a shank portion 31 which carries external threads 32 and projects along axis 24 through a central circular opening 33 in backing plate 27. An enlarged circular head 34 of screw 20 may be received within an enlarged circular opening 35 of the second backing plate 28, and preferably is contained within a recess 36 in an annular sheet metal element 37 which is shaped as shown to bear against both of the plates 27 and 28 in a manner positively preventing screw 20 from being pulled out of those plates. More particularly, a radially outwardly projecting annular flange 38 of sheet metal part 37 may bear against backing plate 28 at a location about head 34 to transmit axial forces from screw 20 to plate 28.

The surface 39 of the backing plate 28 which faces toward sandpaper disc 12 may be planar, and disposed transversely of axis 24', and form in effect a continuation of the outer surfaces 40 and 41 of screw head 34 and element 37. Secured permanently to these surfaces 39, 40 and 41 there is provided a circular resilient cushion 140, having an upper or axially inner circular surface 42 which is planar and disposed transversely of axis 24. This surface 42 is continuously cemented across the entire area of the backing plate structure to surfaces 39, 40 and 41, typically by a suitable rubber cement, such as that sold by Rubatex Corporation, Bedford, Va. 24523, as Rubatex formula 18203. This rubber cement may be of the solvent activated type, which is initially carried by cushion element in a non-tacky condition, and is converted to a tacky and adhesive condition by application of an appropriate solvent to it at the time of adherence of cushion 140 to backing plate 28, element 37 and screw 20. As seen in FIG. 3, the outer circular peripheral edge 43 of cushion 140 is centered about the same axis 24' as are the other parts of the pad, and desirably projects radially outwardly a short distance beyond the outer periphery of backing plate 28.

Cushion 140 is formed of a resiliently deformable elastomeric material, preferably a rubber, and for best results neoprene. Further, it is desirable that the material of cushion 140 be a porous sponge material, and more particularly be a closed pore sponge material, that is, a material in which the individual pores of the sponge do not communicate with one another. The currently preferred optimum material for cushion 140 is therefore a closed pore neoprene sponge material, optimally having a density rating between about 42 and 43.

The axially outer surface or front face 44 of cushion 140 is irregularized to form a large number of projections 45 extending axially toward the work beyond the intermediate recesses 46 formed between the various projections 45. The outer surfaces 47 of the various projections may be disposed essentially transversely of axis 24', and may lie in a common plane 48 disposed transversely of that axis. Similarly, the axially innermost or deepest surfaces 49 within recesses 46 may be disposed transversely of axis 24' and lie in a common transverse plane 50. The axial depth of the recesses inwardly from the extremities 47 of projections 45, that is, the axial distance between planes 48 and 50, should be substantial, and specifically should be at least about .005 of an inch, usually not over about .050 of an inch, and for best results at least about .025 of an inch. As will be apparent from FIG. 2, the projections and recesses 45 and 46 extend continuously across the entire front face 44 of cushion 140, and are all so small in transverse size as to enable the formation of a very large number of such projections and recesses, preferably several hundred of each. It is also found desirable that the area of the projections constitute between about 25 percent and 75 percent of the entire area of the front face 44, with the optimum relationship being one in which the combined area of all of the projections 44 corresponds substantially to the combined area of all of the recesses 46.

In using the pad 11 shown in FIGS. 1 to 6, a person first applies an appropriate sandpaper retaining adhesive or cement to the outer face 44 of cushion 140, and specifically to the surfaces 47 of projections 45 on face 44, and then moves sandpaper dics 12 into engagement with the adhesive (51 in FIG. 5) to secure the sandpaper tightly to the projections. The adhesive 51 may be applied to all of the projections or to only some of them, preferably the former. Any appropriate sandpaper disc retaining adhesive may be utilized for this purpose, such as the type of product known in the trade as feather edging cement, which is a non-hardening permanently tacky contact cement capable of tightly retaining a sandpaper disc and yet permitting the disc to be stripped from the adhesive and from cushion 140 for replacement. One such feather edging cement on the market is that sold as 3-M Feather Edging Cement by Minnesota Mining and Manufacturing Company.

After the sandpaper disc 12 has been adhered to the irregularized surface 44 of cushion 140 in the manner illustrated in FIG. 5, the entire pad 11 may be screwed into and thus secured to head 19 of tool 10, by advancement of the shank of screw 20 into threaded bore 21 of the head. The tool is then actuated to commence powered revolving motion of pad 11 and the carried sandpaper disc 12, and the revolving sandpaper is moved into engagement with a work surface such as that illustrated at 52 in FIG. 6, to sand and smooth that surface. Because cushion 140 bears against sandpaper disc 12 at only the locations of projections 45, and not at the locations of recesses 46,

the forces exerted against the work surface 52 by the tool and operator are localized at the reduced areas of the projections, and thus have the effect of increasing the pressure or force per unit area. That is, a greater force is applied through sandpaper disc 12 and against the work surface at the location of each of the projections than would be applied if cushion 140 had a completely smooth and planar surface bearing against the sandpaper disc. This increased force results in an improved and more rapid cutting action.

Additionally, the provision of the recessed areas 46 in surface 44 of cushion 140 enables the interprojection portions 53 of sandpaper disc 12 (FIG. 6) to deflect slightly upwardly or axially away from the work surface, and to alternately flex upwardly and downwardly at these recessed areas during the sanding operation in a manner attaining a self-cleaning action tending to remove accumulated particles from the abrasive surface of the sandpaper, and thereby prevent that abrasive surface from filling up with such particles and losing its abrasiveness. This selfcleaning action also further increases the rate of cut of the sandpaper on the work surface, and lengthens the effective useful life of sandpaper disc 12 on the tool and pad. The flexure of the resiliently deformable elastomeric material which forms projections 45 of course enhances the self-cleaning and pressure maximizing effects, with a resultant optimum sanding action.

When the sanding disc 12 wears to a condition requiring replacement, it is stripped manually from surface 44 of cushion 140, as indicated at 54 in FIG. 3, and another disc is applied to the cushion for further use of the tool. When necessary, the surface 44 of cushion 140, and all other surfaces of the cushion, can be cleaned freely with lacquer thinner, wash thinner, or any other suitable solvent which may become necessary for removing accumulated adhesive on the surface 44, and no damage will result to cushion 140 because of its closed pore construction. At surface 44 as well as at the other surfaces of the cushion, all of the pores are so closed as to prevent absorption of any of the solvent or other material into the interior of the neoprene or its equivalent, tothus prevent the damage which has been encountered in prior cushions of the open-pore type.

FIGS. 7 through 11 show a variational form of backing ad 11a constructed essentially the same as pad 11 of FIGS. 1 to 6, but differing in the particular respects brought out below. This pad 11a includes a rigid circular plate 28a serving the purpose of plate 28 in FIG. 3, but in this instance desirably being formed of thin sheet metal, such as aluminum, with the outer circular peripheral edge 29a being centered about axis 24a of the pad 11a. Plate 28a may be centrally rein-forced by an externally circular disc 27a, typically formed of fiberglass reinforced resinous plastic material, whose peripheral edge 30a is centered about axis 24a, and with the plates 27a and 28a being rigidly secured together by appropriate rivets 127a spaced evenly about axis 24a. Mounting screw 20a has a circular enlarged flat head 34a received at the forward side of plate 28a, and carrying a threaded shank 31a extending through openings 33a in parts 27a and 28a, and extending along axis 24a. The screw 20a is positively secured to and retained against rotation relative to plate 28a by a rivet 55, extending axially through aligned openings in plate 28a and head 34a of the screw at a location offset from but parallel to axis 24a. The rivet 55 has heads 56 at its opposite ends to secure the parts together. It is found that a single rivet will serve this purpose if utilized in the manner illustrated in FIGS. 9 and 10.

The cushion a of the FIGS. 7 to 11 form of the invention is circular about axis 24a, and is formed of the same type of material and constructed the same as cushion 140 of the first form of the invention, except as to the below discussed differences in the configuration of its two axially facing opposite side surfaces 42a and 44a. With reference first to the surface 42a, it is noted that this surface has a main annular planar portion 57 disposed transversely of axis 24a and extending across almost the entire extent of plate 28a. At the location of head 34a of screw 20a, surface 42a is recessed downwardly or forwardly beyond the plane of main portion 57 of this surface to form a central circular shallow recess 58 dimensioned to just receive head 34a of the screw, as seen clearly in FIG. 4. About the periphery of the external circular edge 29a of plate 28a, cushion 140a has an anannular portion 59 projecting upwardly or rearwardly through an axial distance corresponding to the thickness of plate 28a, to define an annular planar portion 60 of the rear surface 42a of cushion 140a, which portion 60 is flush with and lies in the same transverse plane as the rear surface 61 of plate 28a. At the juncture of portions 57 and 60 of rear surface 42a of cushion 140a, this cushion may have a short axially extending cylindrical surface 61 centered about axis 24a and of a diameter corresponding to the external diameter of peripheral edge 29a of plate 28a, to thus form a recess in the back side of cushion 140a within plate 28a is received in accurately located and confined and centered relation with respect to cushion 140a. Portion 57 of surface 42a is provided with an adhesive of a type utilized on surface 42 of the FIG. 3 arrangement, which adhesive is activated by application of an appropriate solvent thereto, so that plate 28a and its connected parts 27a and 20a may then be placed in position within the recess defined by cylindrical surface 61 of FIG. 9, and the adhesive will upon drying secure parts 28a and 140a permanently together in their accurately located concentric positions.

The front face 44a of cushion 140a may be identical with front face 44 of the cushion of FIG. 3, except that the projections 45a and recesses 46a do not extend radially outwardly to the outermost extremity of cushion 140a. Instead, the cushion is provided about its periphery with an annular planar surface area 62 (FIGS. 8 and 9), which lies in a plane 63 (FIG. 11) disposed transversely of axis 24a. The outer or forward surfaces 64 of projections 45a desirably in this same plane '63, with the inner walls of recesses 46a of course being recessed or spaced upwardly or rearwardly from that plane. The entire forward face 44a, including its projections and recesses and peripheral flat surface 62, are of the same closed pore construction discussed in connection with the first form of the invention. Recesses 46a and projections 45a may extend across the entire front face of the cushion except at the location of the mentioned peripheral surface 62.

In 'using the pad of FIGS. 7 to 11, assume that the pad has been assembled to the condition illustrated in FIG. 9, and that it is desired to utilize sandpaper disc 12a on the pad. An operator first applies a feather edging cement of the type shown at 51 in FIG. 5 to the forward face 44a of cushion 140a, and then places the sandpaper disc 12a in contact with the adhesive for retention on the cushion. The adhesive (shown at 5111 in FIG. 11) is desirably applied annularly along the entire extent of surface 62 of the cushion, so that the sandpaper disc is tightly retained to the cushion along its entire periphery, to prevent the usual tendency for the paper to tear loose at this location. The adhesive may also be applied to the forwardmost surfaces 64 of some or all of the projections 45a, to further enhance the retention of the disc 12a on the pad. During a powered sanding operation, the projections 45a and recesses 46a enable flexure of the sandpaper disc at the recessed locations, as seen in FIG. 6, to attain the previously discussed self cleaning action, and also enable application of an increased localized unit force on the work surface through the sandpaper disc at the location of the projections, to thus produce a fast cutting action. When it is desired to remove disc 12a, it may be easily pulled off of the cushion 140a, and replaced by another disc, with the same adhesive material 51a serving to retain a series of the sandpaper discs to the cushion before it becomes necessary to add additional adhesive to the unit.

As in the first form of the invention, it is desirable in most instances that the projections 45a constitute between about twenty-five percent and seventy-five percent of the entire area of front surface 44a; and for best results the combined area of all of the projections should preferably be approximately equal to the combined area of the recesses 46a.

What is claimed is:

1. A sander pad comprising a backing plate, a fastener carried by said plate at essentially a central location for detachably connecting said plate to a driving unit and for rotation relative to a portion of said unit, a cushion carried by said plate at a forward side thereof and having a front surface to which an abrasive disc is to be removably attached by adhesive, said cushion being formed of a resiliently deformable elastomeric material which is softer than said plate, said front surface of the cushion forming a large number of projections of said elastomeric material to exert localized sanding force thereagainst, and forming a large number of recesses between said projections relieved away from said disc.

2. A sander pad as recited in claim 1, in which said fastener is a mounting screw projecting rearwardly from said plate for threaded connection to said driving unit.

3. A sander pad as recited in claim 1, in which said backing plate is circular.

4. A sander pad as recited in claim 1, in which said cushion is circular.

5. A sander pad as recited in claim 1, in which said cushion is formed of a closed pore elastomeric sponge material.

6. A sander pad as recited in claim 1, in which said cushion is formed of closed pore neoprene sponge material.

7. A sander pad as recited in claim 1, in which said projections extend outwardly between about .005 of an inchand .050 of an inch beyond the deepest portions of said recesses.

8. A sander pad as recited in claim 1, in which the area of said projections is between about 25 percent and 75 percent of the entire area of said front surface.

9. A sander pad as recited in claim 1, including adhesive on said front surface for attaching said abrasive disc thereto.

10. A sander pad as recited in claim 1, in combination with an abrasive disc extending across said front surface of the cushion, and adhesive securing said disc to said front surface.

11. A sander pad as recited in claim 1, in which said fastener is a mounting screw projecting rearwardly from said plate for threaded connection to said driving unit, said plate and cushion being essentially circular and centered essentially about and disposed transversely of the axis of said screw, said cushion being of a diameter greater than said plate.

12. A sander pad as recited in claim 11, in which said cushion is formed of closed pore neoprene sponge material, the area of said projections being between about 25 percent and percent of the entire area of said front surface, said projections extending outwardly at least about .025 of an inch beyond the deepest portions of said recesses.

13. A sander pad as recited in claim 12, in combination with an abrasive disc extending across said front surface of the cushion, and adhesive securing said disc to said projections on said front surface.

14. A sander pad as recited in claim 1, in which said front surface of the cushion has a portion extending essentially along its periphery and which is disposed essentially about and is smoother than the portion of said surface which has said projections and recesses.

15. A sander pad as recited in claim 1, in which said front surface of the cushion has an essentially annular peripheral portion which is disposed about the portion of said surface having said projections and recesses and which is essentially fiat and essentially aligned transversely with the extremities of said projections.

16. A sander pad as recited in claim 1, in which said cushion has a back surface containing a rearwardly facing recess within which said plate is received in a closely fitting relation locating and centering the plate relative to said cushion.

17. A sander pad as recited in claim 16, including adhesive securing said cushion to said plate within said rearwardly facing recess.

References Cited UNITED STATES PATENTS 529,510 11/1894 Therrien 51377 1,693,163 11/1928 Schacht 51--17() 2,990,661 7/1961 Hackett 51-379 3,014,319 12/1961 Olton 5l358 3,082,582 3/1963 Jeske 5l-358 OTHELL M. SIMPSON, Primary Examiner US. Cl. X.R. 51358 

